Probe system

ABSTRACT

A probe system for use in carrying out inspections in an enclosed, non-conducting environment, such as an endoscope for use in carrying out internal inspections of a human or animal patient, comprises sensing coils (7) mounted at spaced positions along the probe (6) for movement with the latter and a planar grid array (1) of antennas (2) which is disposed adjacent the environment. AC electrical signals are supplied to the antennas, either simultaneously or sequentially, by an electrical source (4,5) so as to induce corresponding voltage signals in the sensing coils (7). When the probe (6) is introduced into the environment, the induced signals, converted to a digital format, are processed by a microprocessor (12) to produce a visual indication of the three dimensional location of the probe (6) with respect to the antenna array (1).

This invention relates to probe systems for use in an enclosed,non-conducting environment, such as a human or animal body. Such probesystems are particularly suitable for use in endoscopy and, moreparticularly, colonoscopy. The invention also relates to a method oflocating a probe within the body of a human or animal subject.

It is of particular importance to determine the precise location of aprobe, such as an endoscope, within the body of a patient. The absenceof an accurate probe locating system can cause patient complicationsranging from unnecessary pain to actual perforation, instrument damage,and serious mistaken diagnosis. A further complication is that elongateprobes have a tendency to form looping configurations. One probelocating technique is to employ x-ray imaging, but x-ray systems sufferfrom several disadvantages. They are relatively expensive and, hence,are not readily available in most endoscopic units. In addition, x-raysystems now face increasing restrictions on their use, such asprotective clothing, etc. Furthermore, they produce only a relativelysmall two dimensional image, which is somewhat lacking in detail.

It is an object of the present invention to provide a probe system whichis capable of being accurately located within an enclosed non-conductingenvironment, such as a human or animal body, and which does not rely onx-ray imaging for its locational information.

Accordingly, the invention consists in a probe system for use in anenclosed, non-conducting environment and including a probe having atleast one sensing coil movable with the probe, an antenna array, anelectrical source for supplying the antenna array with an AC electricalsignal for inducing a corresponding electrical signal in the or eachsensing coil, and electronic processing means connected to the sensingcoil(s) for processing the induced signals and producing an indicationof the three dimensional location of the probe with respect to theantenna array, characterised in that the antenna array has two sets ofdipole antennas comprising elongated coils arranged in a substantiallyplanar and mutually orthogonal array, and the electrical source isadapted to supply the AC electrical signal to each antenna coil eithersimultaneously or sequentially.

The invention makes use of the variation in mutual inductance between asensing coil and the antenna array. By detecting the signals induced inthe or each sensing coil by each transmitting antenna of the array, andsolving field equations associated with each antenna location, the threedimensional location of the sensing coil and, hence, the probe, can beestablished.

In one embodiment of the invention, the sensing coil is mounted to movewith the probe, which has coordinates X,Y,Z and polar angles θ, φ, andthe planar arrays or grid of mutually orthogonal antenna coils ismounted adjacent the enclosed non-conductive environment to be accessedby the probe with the coils of the array running respectively parallelto the X and Y axes. When a single antenna coil of the grid carries anAC current signal, a voltage is induced in the sensing coil which variesin a completely defined manner with respect to the relative coordinatesand the sensing coil angles. When each antenna coil of the grid isenergised in turn with a short current pulse supplied from theelectrical source, the sequence of signals produced by the sensing coilmay be analysed to yield the sensing coil coordinates and angles. Whenused in endoscopic applications, the antenna array is mounted on thepatient bed underneath a patient with the coils of the array runningrespectively parallel to the X and Y axes.

A probe, such as an endoscope, typically comprises an elongate element,in which event information concerning the location of various sectionsof the probe may be required. Moreover, in addition to the positionalinformation, it may be desirable to detect the occurrence of accidentalloops formed in the probe behind its tip. In one embodiment, at leastone sensing coil is movable longitudinally with respect to the probealong at least a portion of its length. By moving the sensing coil alongthe probe and detecting its successive positions, the existence oflooped configurations or other problems may be detected. Alternatively,two or more sensing coils may be mounted in spaced relationship alongthe length of the probe, one preferably being located adjacent the tipof the probe and the others remote from the tip. By employing such aplurality of sensing coils, data concerning the location of each portionof the probe can be obtained. Moreover, even with only two sensingcoils, the relative phase of the induced signal in the two sensors, whena single antenna in the array is energised, provides information about atwist along the length of the probe and thus can be monitored to givewarning of an impending loop formation. The exact location of a loop maybe detected by mounting several sensors along the length of the probe.Changes in relative phase will then pinpoint which sections are twistedand, hence, looped.

With the invention, having obtained three dimensional coordinates of atleast one point on the probe and at least two known points on thesurface of the enclosed non-conducting environment, the electronicprocessing means may operate to compute an estimate of the position ofthe probe inside the environment. This task falls into two parts,namely, interpolating the known points to find the centre line of theprobe and, secondly, estimating the tip position in the environment. Thefirst task is relatively trivial if sufficient points are known whilstthe second task is more difficult. For example, in the case ofcolonoscopy, although there is a great deal of variation in individualcolons, there are certain anatomical properties that can be used. Hence,the sigmoid and transverse colons are known to mobile, whereas theascending and descending colons are relatively fixed. Thus, by lookingat the dynamic behaviour of different points on the endoscope, theposition in the colon can be estimated. For example, if a point moves ina direction orthogonal to the endoscope body it is probable that thatpoint is either in the sigmoid or transverse colon. Evidence can also begained from the configuration of various parts of the endoscope. Furtherinformation is available from the inserted length of endoscope. Thisinformation must be incrementally upgraded since manoeuvres, such aspulling back to straighten the sigmoid colon, alter the relationshipbetween the inserted length and the position of the endoscope tip. Thealgorithm for computing the probe position may be one in which theuncertainty is as far as possible resolved by combining several sourcesof information. Once the position of the tip is estimated, it is acomparatively simple matter draw the endoscope and indicate thesurrounding haustral folds.

The electrical source is preferably adapted supply the antennas with ACsine wave signals having frequencies in the low to medium audio ranges,that is 1 to 10 kHz and, preferably, of the order of 2 kHz.Alternatively, the generator may be adapted to supply the antennas withradio frequency AC electrical signals.

In one convenient embodiment of the invention, for use with anendoscope, a plurality of sensing coils, for example seven, and theirconnecting leads are mounted at intervals inside a flexible protectiveplastics sheath, for example, a plastic tube approximately 2 m in lengthand having an external diameter less than 2.5 mm, which allows insertionof the coils into the biopsy channel of the endoscope. Location andimaging of the entire length of the scope inside a patient can thus beobtained on an unmodified endoscope by inserting the sheath into thebiopsy channel and operating the sensing coil energisation sequence. Inanother embodiment requiring modification of a conventional endoscope,sensing coils are permanently mounted at intervals along the length ofthe scope. These coils are mounted substantially coaxially with theendoscope axis and are disposed between the metallic braid and the outerplastics sheath of the endoscope. With this embodiment, continuousmonitoring of the endoscope location is obtained without any obstructionto the biopsy channel.

The or each sensing coil may be connectable to the electronic processingmeans via a preamplifier, a phase sensitive detector, which produces ademodulated DC analogue voltage correspondingly to the AC voltage signalinduced in the sensing coil by the antenna array, and ananalogue-to-digital (A/D) converter which digitises the coil signal forprocessing by the electronic processing means to produce a finalgraphical image. Where the probe system includes a plurality of sensingcoils, each sensing coil may have its own preamplifier, phase sensitivedetector and A/D converter chain or, alternatively, a multiplexingarrangement may be utilised for sequentially detecting the signalvoltages induced in the sensing coils and utilising a singlepreamplifier, phase sensitive detector and A/D converter chain.

The antenna array is in the form of a substantially planar grid ormatrix with some or all of the antenna of the array comprising dipoleantennas. The use of dipole antennas reduces the signal contributionfrom the wire leads and the return wire.

The planar array of antennas may, for example, comprise two orthogonalsets of seven or more long thin dipole coils. Each set provides enoughdata to compute the Z (height) and either X or Y components of the oreach sensing coil position with respect to the array. This configurationin conjunction with the electronic processing means allows for a rapidcomputation of sensing coil position because the X and Y coordinates areseparately determined.

Small errors in position may be produced by dipole coils having plain,short ends because these sections on, for example, the Y coils, producesmall but defined contributions to the X component of the field andthese contributions are not taken into account in the computationproduced by the electronic processing means. In order effectively toreduce these contributions, the ends of the coil turns may be speciallyconfigured as so called "butterfly" end windings which alter aneffective short single wire section into a dipole perpendicular to thelongitudinal axis of each coil. The field from such an arrangement fallsoff with distance more rapidly than that of a single wire and thus thefield contribution of the ends is very much reduced.

The electrical source for supplying AC electrical signals to the antennaarray may be adapted simultaneously to energise each of the plurality ofantenna coils with a distinct or individual frequency. In this way, thetime taken to establish the location of the probe may be reduced as allthe antenna coils are transmitting simultaneously, each with its owndistinctive frequency. Where the time taken to interrogate the or eachsensing coil is not critical, the electrical source may operate at asingle frequency energising each of the antenna coils in turn.

The present invention also consists in a method of locating a probewithin the body of a human or animal subject, comprising the steps ofinserting the probe into the body of the subject, said probe having atleast one sensing coil movable therewith, disposing an antenna arrayadjacent the subject, said antenna array having two sets of dipoleantennas comprising elongated coils arranged in a substantially planarand mutually orthogonal array, energising each of the antenna coils,either simultaneously or sequentially, with an AC electrical signal,detecting the resulting signal induced in the or each sensing coil, andprocessing the detected signal(s) to produce an indication of the threedimensional location of the probe with respect to the antenna array.

Preferably, the method includes the further step of detecting thesignals induced in one or more reference coils externally of the body ofthe subject, in order to establish the location of the body of thesubject with respect to the antenna array.

In order that the present invention may be more readily understood,reference will now be made to the accompanying drawings, in which:

FIG. 1 schematically illustrates one embodiment of the invention,

FIG. 2 schematically illustrates the winding configuration of eachdipole antenna of the antenna grid,

FIG. 3 is a diagram utilised in applying a correction to the fieldequations in order to take into account the fact that the antenna wiresare of finite length, and

FIG. 4 is a graph illustrating the variation in the two components ofthe equation on which is based the algorithm for solving the fieldequations.

Referring to FIG. 1 of the drawings, the probe system includes an array1 of dipole antennas 2 in the form of a planar grid comprising twomutually orthogonal sets of seven elongated thin coils extending in Xand Y directions. Each dipole antenna 2 may, for example, comprise acopper coil having twenty turns and typically have dimensions of 100cm×5 cm. The spacing between the antennas 2 within the grid array 1 istypically 7.5 cm.

As illustrated in FIG. 2, the ends 3 of each turn of a dipole antennacoil 2 are specially configured. The reason for this is that smallerrors in position would be produced by coils with plain short endsbecause these sections, for example, on the Y coils produce small butdefined contributions to the X component of the field. By providing theturns with so-called "butterfly" end windings, as illustrated, theseundesirable field contributions are effectively reduced. In effect, thisarrangement alters a short single wire section at each end into a netdipole perpendicular to the longitudinal axis of the associated coil.

Each antenna 2 is connected via twisted leads 2a to a relay box 4 whichis adapted sequentially to connect each antenna with an AC signalgenerator 5. In one example, the generator 5 is capable of generating a9 kHz sine wave signal having a peak current of 50 mA.

Associated with the antenna array 1 is an endoscopic probe 6 havingseven sensing coils 7 mounted coaxially with the probe at spacedpositions along its length, one of the sensing coils being mounted at oradjacent the tip of the probe. For the sake of convenience, only twocoils 7 are illustrated. Each sensing coil 7 is wound about the probebetween the metallic braid and the outer plastic sheath of the probe andtypically comprises 800 turns of fine copper wire (48 swg) on a tubularMu metal core which is a commercially available soft ferro magneticmaterial with a high relative permeability (in the range10,000-100,000). Each sensing coil 7 is connected via a screened twistedcable pair 8 to a preamplifier 9 which is in turn connected to a phasesensitive detector 10 for producing a DC voltage level proportional tothe amplitude of the AC voltage induced in the associated sensing coil7. The signals from the phase sensitive detector 10 are fed to an A/Dconverter 11 which produces digital signals dependent on the DC signallevel at the output of the phase sensitive detector for processing by amicroprocessor 12 and subsequent display on a video display unit 13associated with the microprocessor. Only the chain of circuits 9,10 and11 associated with one sensing coil 7 is illustrated in FIG. 1.

In using the system of FIG. 1 for conducting an endoscopic inspection ofa cavity or passage, for example, the colon, in the body of a human oranimal patient, the planar antenna array 1 is disposed underneath thepatient on the patient's bed. Conveniently, the antenna array is formedby winding the multi-turn coils of the antennas 2 in grooves cutaccurately in a single large board that forms part of the patient's bed.Thus, the array is permanently in place and does not hinder thepositioning and turning of the patient during an endoscopic procedure.Having positioned the patient on the antenna array, the endoscopic probe6 is inserted into the colon of the patient, whereafter, each antenna 2is energised in turn by the AC electrical source constituted by therelay box 4 and generators. The resulting signals induced in the sensingcoils 7 are fed to the microprocessor 12 via the chain of circuits 9, 10and 11 and the microprocessor interprets the signals in terms of thethree dimensional locations of the probe. The X,Y and Z coordinates andthe axial direction vector of each coil 7 can be established bycomparing the signals induced from each antenna 2 and by solving certainfield equations associated with each antenna location.

Additional sensing coils (not shown) may be placed in predeterminedpositions on the patient's body (body markers) so that the position ofthe torso can be estimated with respect to the array 1 which serves as acommon reference frame. By this means, the position and configuration ofthe endoscope with respect to the patient's anatomy can be computed.

The determination of sensing coil positions by the microprocessor relieson the integral equation for the mutual inductance between two coils.For a general coil arrangement the equation (Von Neumann's formula) doesnot allow the direct determination of sensing coil coordinates frommeasurement of mutual inductance. For the purposes of the presentinvention, therefore, this is simplified in the following ways:

(i) Each sensing coil 7 has a small area and thus it can be consideredto be a point magnetic induction detector.

(ii) The antenna array 1 separates the determination of the X and Ycomponents. For each set of dipoles 2 (X and Y) the signal isinsensitive to sensing coil position along the longitudinal axis of eachset.

(iii) The width of each coil (dipole is sufficiently small) that thefield produced by each antenna can, to a good approximation beconsidered as the derivative with respect to wire location of the fielddue to a long single wire.

Even with these simplifications there is no exact analytic method forthe determination of X, Y, Z coordinates of a sensing coil. Ratherapproximate iterative schemes have to be employed. Applicants method hastwo steps:

(a) The first step makes use of the spatial properties of the functionsdescribed below in order to get a first estimate of the X,Y,Zcoordinates from the mutual inductance measurements associated withrespectively the X and Y dipole sets. This very much reduces the rangeover which the second stage has to search in order to fit the modelequations to the data.

(b) In the second step, an iterative algorithm searches the reducedregion of the X,Y axes checking at each stage that the revised estimatesprovide a better fit to the data.

In this scheme two independent estimates of Z are obtained and these canbe used as a check on the accuracy of the final estimates.

The following is a more detailed description of steps (a) and (b).

Hence, the equations to be solved are:

The equation for the mutual inductance between an infinitely longstraight wire, parallel to the Y axis at coordinate xi, and a coil 7 atcoordinate x,y,z is:

    M.sub.i =(n.sub.x z+n.sub.z (x.sub.i -x))/((x.sub.i -x).sup.2 +z.sup.2)(1)

where [n_(x),n_(y),n_(z) ] is a vector in the axial direction of thecoil whose magnitude is dependent on the current in the wire, and thesize and number of turns of the coil. A similar equation relates mutualinductance to position for wires parallel to the X-axis.

For the case of a dipole 2 replacing the infinite wire, we make theapproximation that the mutual inductance is the differential of theequation for magnetic flux induced in coil B when a current i flows in acoil A (that is φ_(A) =M_(AB) i_(B)) with respect to x, which yields

    M.sub.i '=(2n.sub.x z(xi-x)+n.sub.z ((x.sub.i -x).sup.2 -z.sup.2))/((x.sub.i -x).sup.2 +z.sup.2).sup.2            (2)

It is also convenient to consider the second differential of Mi, sincethe stationary points of the dipole equation are of interest. It is:

    M.sub.i "=(2n.sub.x z(3(xi-x).sup.2 -z.sup.2)+2n.sub.z (x.sub.i -x)((x.sub.i -x).sup.2 -3z.sup.2))/((x.sub.i -x).sup.2 +z.sup.2).sup.3(3)

The above equations assume that the wire length is infinite. However, inany practical application finite wires are used. A correction can bemade to account for the fact that the induced voltage is less. Hence,referring to FIG. 3, if α1 and α2 are the angles between lines joining acoil 7 to the ends of the transmitting wire and the perpendicular fromthe coil to the wire, then the induced voltage is reduced by:

    2/(Sin(α1)+Sin (α2))                           (4)

A direct solution to the single wire equation is available. It is foundby multiplying out equation (1) to give:

    2M.sub.i x.sub.i x-M.sub.i (z.sup.2 +x.sup.2)+x.sub.i n.sub.z +n.sub.x z-n.sub.z x-M.sub.i x.sub.i.sup.2 =0                      (5)

if we make the substitutions:

    s1=(z.sup.2 +x.sup.2)                                      (6)

    s2=(n.sub.x z-n.sub.z x)                                   (7)

we obtain a linear equation in four variables, x,nx,s1 and s2, which canbe solved with four readings (i=0,1,2,3) from four wires. The values ofz and nz can be obtained from the simple quadratic equations (6) and (7)after solution of the four linear equations.

An experimental system has been devised to try this solution method,using an area of 100 sq. cm., and it was found to work well for valuesof z greater than 5 cm. It is believed that with further attention tothe computation a stable solution could be obtained by this method overthe required operating range.

Since this method depends critically on the relative magnitudes of thereadings taken from the individual wires, the finite length α correctionfactors must be applied. Addition of these correction factors raises theorder of this solution such that it becomes incomputable directly.However, an iterative scheme can be applied simply, as follows:

(i) Compute an estimate of the position of a sensing coil 7 using thedirect linear solution.

(ii) For each measurement, compute the α correction based on the coil'sestimated position. Correct the measured value by the inverse of thisfactor.

(iii) Recompute the estimate of the sensing coil position.

(iv) If the new estimate is sufficiently close to the old estimate thenterminate, otherwise go to step (ii).

This algorithm has been found to work satisfactorily with simulateddata. However, further investigation has been suspended in view of thebetter results obtained using dipole antennas 2 rather than singlewires.

Although there are only four unknowns in the dipole equation (2), thefourth order terms means that a direct solution is not available fromfour sets of readings.

The fourth order terms in the dipole equation means that, despite thefact that there are only four unknowns, a solution cannot be obtaineddirectly from four measurements. Either an iterative solution must beused, or an over specified system of equations is required. The lattermay be done by the same method outlined above. Multiplying out thedipole equation, and grouping the terms with common coefficients, weobtain:

    M.sub.i x.sub.i -4M.sub.i x.sub.i.sup.2 (3x.sup.2 +z.sup.2)-4M.sub.i x.sub.i (x.sup.3

     +xz)+M.sub.i (x.sup.4 +z.sup.4 2x.sup.2 z.sup.2)-x.sub.i.sup.2 nz+2x.sub.i (n.sub.z x-n.sub.x z)+(2n.sub.x zx+n.sub.z x.sup.2)=

     0                                                         (8)

as before we can linearise equation (3) by substituting:

    s1=(3x.sup.2 +z.sup.2)                                     (9)

    s2=(x.sup.3 +xz)                                           (10)

    s3=(x.sup.4 +z.sup.4 +2x.sup.2 z.sup.2)                    (11)

    s4=(n.sub.z x-n.sub.x z)                                   (12)

    s5=(2n.sub.x zx+n.sub.z x.sup.2)                           (13)

to obtain a system of equation in seven variables (x,nz,s1,s2,s3,s4,s5). After solution values for z and nx may be obtained fromfurther solution of equations (9-13). This solution has not been testedfor the following reasons. Firstly, since it depends on the magnitudesof the seven measured points, it is necessary to correct for the finitelengths of the dipoles. This requires an iterative process whichincludes a gaussian elimination on a seven by seven matrix and,consequently, is computationally very slow. Secondly, the presence offourth order terms makes the method less robust against error.

Accordingly, a fast iterative algorithm has been developed for solutionof the field equations. It can be formulated for both single wires ordipoles. By way of example, the latter form is presented below.

The algorithm is based on the fact that the equation is a linearcombination of two components:

    2z (xi-x)/((x.sub.i -x).sup.2 +z.sup.2).sup.2              (14)

and

    (x.sub.i -x).sup.2 -z.sup.2))/((x.sub.i -x).sup.2 +z.sup.2).sup.2(15)

These are shown in FIG. 4. The nz component has a peak at the positionof the coil and zeros at a distance of z from the peak, and the nxcomponent has a zero at the same place, and two peaks at a distance z√3from the position of the coil. In FIG. 4, the curves are drawn assumingpositive nx and nz. Summing these two curves, the resulting graph musthave a zero to the right of the peak, at the point where the positive(nx) component equals the negative (nz) component. This zero must occurat a distance of less than, z from the position of the coil. The summedgraph must also have a peak to the left of the coil position. It willoccur when the negative slope of the nz-component is equal to thepositive slope of the nx-component. This will be somewhere between thecoil position, where the slope of the nz component is zero, and thenegative peak of the nx trace, where its slope falls to zero. Hence, thepeak is at most z√3 from the coil position. The summed curve will alsohave a smaller peak at a distance of greater than z to the right of thecoil, and possibly another peak and zero to the left of the coil. It isclear to see that (for nz>0) these peaks must be smaller in magnitudethan the one close to the root close peak, since the amplitude of bothtraces fall off sharply with distance from the coil position. Similarly,it is clear to see that the other zero, if present, is further from themain peak. Hence, for a give set of measurements, it is known that thecoil is located between the largest peak and its closest zero, which arecloser together than z(1+1/√3). Similar arguments apply to the caseswhere nx and nz are both negative, or of different signs.

The algorithm is a binary search of the space between the largest peakand its closest zero. If an estimate is made of the coil position, sayxc, and we write:

    uz=xc-xz                                                   (16)

    up=xc-xp                                                   (17)

where xz and xp are the positions of the peak and the zero, then thefollowing equations result:

    Mg=n.sub.z /z.sup.2                                        (18)

    Mp=(2n.sub.x z up+nz(up.sup.2 -z.sup.2))/(up.sup.2 +z.sup.2)(19)

    2n.sub.x z uz=-n.sub.z (uz.sup.2 -z.sup.2)                 (20)

    2n.sub.z z(3up.sup.2 -z.sup.2)=2n.sub.z up(up.sup.2 -3z.sup.2)(21)

For the most accurate estimate, equations (18), (20) and (21) can besolved to provide an estimate for nx,nz and z. The resulting graph isthen compared with the real data. For points outside the peak to nearestzero span, an estimate too close to the zero results in the estimatedmagnitudes being too high, and vice versa, which gives a simplecriterion for choosing how to improve the estimate xc, by binary search.Equation (19) was used as an alternative to equation (21) in the currentstudy. It has the advantage that the position of the peak need not beknown, any magnitude will suffice. However, it produces a less accuratesolution, since it depends on a measured magnitude, at or near to thepeak, and hence requires compensation for the finite dipole lengths.Note that the positions of the peaks and zeros are not altered by thedipole lengths.

Two problems have been found with the method described above. Firstly,in some cases it is possible that the closest zero is not within themeasured range. This can be partly solved by extrapolation of themeasured data, but this technique is error prone, and thereforeundesirable. If the measured range is extended by the maximum operatingheight beyond the position range in which the coil is to be located,then this problem will never arise. This is possible for the endoscopeapplication in one dimension. Having solved for that direction, use canbe made of the known value of z to provide a more accurate extrapolationin the other direction.

The second problem is caused when the values of nx and nz are both closeto zero. In this case very low measurements are obtained. This conditioncan be identified, since in the orthogonal direction the trace has aclearly defined zero with symmetrical peaks. One possible solution inthis case is to use an alternate set of coils, configured to provide thelocalisation for this special case. Since everything about the coil,except its x (or y) coordinate is known, this arrangement can be keptvery simple.

Whilst a particular embodiment has been described, it will be understoodthat modifications can be made without departing from the scope of theinvention, as defined by the appended claims. For example, whilst aseven by seven antenna array 1 is shown and described, larger planargrid arrays are possible. Moreover, instead of mounting the sensingcoils 7 at fixed positions about the endoscopic probe, one or more coilswith connecting leads for coupling to associated preamplifiers may bedisposed at intervals inside a protective plastics sheath capable ofbeing inserted and moved longitudinally of the probe along the biopsychannel of the endoscope. Instead of providing each sensing coil 7 withits own individual chain of circuits 9, 10 and 11, a multiplexing systemmay be used sequentially to connect the sensing coils to a single suchchain. The microprocessor 12 may control switching of the relay box 4 toproduce energisation of the dipole antennas 2 of the array 1 in apredetermined sequence.

We claim:
 1. In a probe system for use in an enclosed, nonconductingenvironment and including a probe having at least one sensing coilmovable with said probe, an antenna array, an electrical source forsupplying said antenna array with at least one AC electrical signal forproducing corresponding induced electrical signals in said at least onesensing coil, and electronic processing means connected to said at leastone sensing coil for processing said induced signals and producing anindication of the three-dimensional location of said probe with respectto said antenna array, the improvement comprising said antenna arraywherein said array has two sets of dipole antennas comprising elongatedcoils arranged in a substantially planar and mutually orthogonal array,and said electrical source includes means for supplying said at leastone AC electrical signal to said antenna coils either simultaneously orsequentially.
 2. A probe system according to claim 1, wherein saidelectrical source includes means for supplying said antenna coils withat least one audio frequency AC electrical signal.
 3. A probe systemaccording to claim 1 or 2, wherein said probe is an elongated elementand said at least one sensing coil includes a plurality of sensing coilsdisposed at spaced positions along the length of said probe.
 4. A probesystem according to claim 1, wherein said probe is an endoscope having abiopsy channel and said at least one sensing coil is disposed in thebiopsy channel of said endoscope.
 5. A probe system according to claim1, wherein said at least one sensing coil is mounted on said probesubstantially coaxially therewith.
 6. A probe system according to claim1, including phase sensitive detector means for producing a demodulatedDC analog voltage proportional to said induced electrical signals insaid at least one sensing coil, and said at least one sensing coil isconnected to said processing means via said phase sensitive detectormeans.
 7. A probe system according to claim 6, including preamplifyingmeans connecting said at least one sensing coil to said phase sensitivedetector means, and A/D converting means connecting said phase sensitivedetector means to said processing means for converting said DC voltageproduced by said detector means into digital signals for processing bysaid processing means.
 8. A probe system according to claim 1, whereinsaid electrical source includes means for simultaneously supplying eachsaid antenna coil of said antenna array with an AC electrical signal ofdistinct frequency.
 9. A method of locating a probe within the body of ahuman or animal subject, comprising the steps of inserting said probeinto the body of said subject, said probe having at least one sensingcoil movable therewith, disposing an antenna array adjacent saidsubject, said antenna array having two sets of dipole antennascomprising elongated coils arranged in a substantially planar andmutually orthogonal array, energizing said antenna coils, eithersimultaneously or sequentially, with at least one AC electrical signal,detecting resulting signals induced in said at least one sensing coil,and processing said detected signals to produce an indication of athree-dimensional location of said probe with respect to said antennaarray.
 10. A method according to claim 9, further comprising the step ofelectronically establishing the position of the body of said subjectwith respect to said antenna array thereby to enable computation of theposition and configuration of the probe with respect to said body.